1. Field of the Invention
The present invention relates to a velocity control device by which a laser beam spot on an optical disk, for example, is moved to a desired track.
2. Description of the Related Art
When data is recorded on an optical disk, or when data recorded thereon is read, for example, a seek operation in which a laser beam spot is moved to a desired track is carried out, and to obtain an optimum performance of an optical disk device, this seek operation should be carried out at a high speed and with a high precision. Namely, in the optical disk device, the speed at which a beam spot is moved to a desired track must be high, and the beam spot must always arrive at the desired track.
Since the speed at which the laser beam spot is moved is determined by the characteristics of a drive circuit provided for operating an actuator of the optical head, the laser beam spot is moved at a finite acceleration. Therefore, when the laser beam spot is moved at a constant acceleration, to stop the laser beam spot with a high accuracy at a desired track, the laser beam spot should be moved at a velocity proportional to a square root of a distance from a present position of the laser beam spot to the desired track position. Namely, a function showing a velocity profile of the laser beam spot is EQU f(.epsilon.)=(2.alpha..epsilon.).sup.1/2 ( 1)
wherein .alpha. is an acceleration, and .epsilon. is a distance from the present position of the laser beam spot to the desired track position.
As described above, by controlling the velocity of the laser beam spot in accordance with the equation (1), a precise seek operation is obtained. Such a control can be attained by forming a velocity servo loop including an actuator and a control circuit provided for moving the laser beam spot.
Nevertheless, since the actuator included in the velocity servo loop is a mechanical element, a gain and a bandwidth of the velocity servo loop cannot have a large value, and thus it is difficult to obtain a high responsivity. Therefore, a precise laser beam spot movement cannot be made by using the velocity servo loop alone, and thus it is difficult to attain a precise seek operation.